Steerable focal adjustment for optical coherence tomography

ABSTRACT

A system and method for surface inspection of an object using optical coherence tomography (OCT) is provided. The method includes determining a surface profile of the object, the surface profile includes one or more regions on a surface of the object; moving the object relative to the OCT scanner head; and for each of the one or more regions on the surface of the object, performing: determining a working distance where the surface of the object at the respective region is within a present depth of field; determining an angle where the respective region is at the determined working distance from an OCT scanner head; directing the OCT scanner head at the determined angle towards the respective region when the respective region is at the determined working distance along the respective angle; and performing an A-scan of the object when the respective region is within the present depth of field.

TECHNICAL FIELD

The following relates generally to imaging and more specifically to asystem and method for steerable focal adjustment for optical coherencetomography.

BACKGROUND

In many applications, imaging can be used to garner information about aparticular object; particularly aspects about its surface or subsurface.One such imaging technique is tomography. A device practicing tomographyimages an object by sections or sectioning, through the use of apenetrating wave. Conventionally, tomography can be used for variousapplications; for example, radiology, biology, materials science,manufacturing, quality assurance, quality control, or the like. Sometypes of tomography include, for example, optical coherence tomography,x-ray tomography, positron emission tomography, optical projectiontomography, or the like.

Conventionally, the above types of tomography, and especially opticalcoherence tomography, produce detailed imaging of an object; however,inaccuracies and problems can arise with respect to properly imaging theobject.

SUMMARY

In an aspect, there is provided a method of surface inspection of anobject using optical coherence tomography (OCT), the method comprising:determining a surface profile of the object, the surface profilecomprising one or more regions on a surface of the object; moving theobject relative to the OCT scanner head; and for each of the one or moreregions on the surface of the object, performing: determining a workingdistance where the surface of the object at the respective region iswithin a present depth of field; determining an angle where therespective region is at the determined working distance from an OCTscanner head; directing the OCT scanner head at the determined angletowards the respective region when the respective region is at thedetermined working distance along the respective angle; and performingan A-scan of the object when the respective region is within the presentdepth of field.

In a particular case, moving the object relative to the OCT scanner headcomprises continuously moving the object.

In another case, the one or more regions on the surface of the objectcomprise one or more regions in a first section of the surface profile,the surface profile comprising at least one subsequent section, eachsubsequent section comprising one or more subsequent regions, the methodfurther comprising, for each one of the one or more subsequent regions,performing: determining a working distance where the surface of theobject at the respective subsequent region is within the present depthof field; determining an angle where the respective subsequent region isat the determined working distance from the OCT scanner head; directingthe OCT scanner head at the determined angle towards the respectivesubsequent region when the respective subsequent region is at thedetermined working distance along the respective angle; and performingan A-scan of the object when the respective subsequent region is withinthe present depth of field.

In yet another case, at least a portion of the object has a curvedsurface profile.

In yet another case, determining the surface profile of the objectcomprises measuring a plurality of distances between a fixed point andthe surface of the object.

In yet another case, the method further comprising retrieving a surfacegeometry of the object from a database and wherein determining thesurface profile of the object comprises determining a distance betweenthe scanner head and the surface of the object from the surface geometryof the object.

In yet another case, the one or more regions are approximately 100 to1000 microns away from an adjacent region.

In yet another case, the one or more regions are approximately 200microns away from the adjacent region.

In yet another case, the angle where the respective region is at thedetermined working distance is between zero-degrees and ninety-degreesin a direction of movement of the object.

In yet another case, the angle where the respective region is at thedetermined working distance is between zero-degrees and ninety-degreesin a direction opposite to a direction of movement of the object.

In yet another case, the angle where the respective region is at thedetermined working distance is between zero-degrees and ninety-degreesin a direction of movement of the object for regions located on downwardsloping portions of the surface profile, and the angle where therespective region is at the determined working distance is betweenzero-degrees and ninety-degrees in a direction opposite to the directionof movement of the object for regions located on upward sloping portionsof the surface profile.

In an aspect, there is provided a system for surface inspection of anobject using an optical coherence tomography (OCT) system, the OCTsystem comprising an optical source to produce an optical beam, a beamsplitter to direct a derivative of the optical beam to a reflectiveelement and another derivative of the optical beam to the object via ascanner head and direct optical beams returned from the reflectiveelement and the object to a detector for detection of an interferenceeffect, the system for surface inspection comprising: a distancedetermination module to determine a surface profile of the object, thesurface profile comprising one or more regions on a surface of theobject; an object translator to move the object relative to the OCTscanner head; and a steering module to, for each of the one or moreregions on the surface of the object, perform: determining a workingdistance where the surface of the object at the respective region iswithin a present depth of field; determining an angle where therespective region is at the determined working distance from an OCTscanner head; and directing the OCT scanner head at the determined angletowards the respective region when the respective region is at thedetermined working distance along the respective angle, the OCT systemperforming an A-scan when the respective region is within the presentdepth of field.

In a particular case, the distance determination module comprises aplurality of measurement devices to measure a plurality of distancesbetween one or more fixed points and the surface of the object togenerate the surface profile.

In another case, the one or more regions on the surface of the objectcomprise one or more regions in a first section of the surface profile,the surface profile comprising at least one subsequent section, eachsubsequent section comprising one or more subsequent regions, thesteering module, for each one of the one or more subsequent regions,further performing: determining a working distance where the surface ofthe object at the respective subsequent region is within the presentdepth of field; determining an angle where the respective subsequentregion is at the determined working distance from the OCT scanner head;and directing the OCT scanner head at the determined angle towards therespective subsequent region when the respective subsequent region is atthe determined working distance along the respective angle, the OCTsystem performing an A-scan when the respective region is within thepresent depth of field.

In yet another case, the angle where the respective region is at thedetermined working distance is between zero-degrees and ninety-degreesin a direction of movement of the object.

In yet another case, the angle where the respective region is at thedetermined working distance is between zero-degrees and ninety-degreesin a direction opposite to a direction of movement of the object.

In yet another case, the angle where the respective region is at thedetermined working distance is between zero-degrees and ninety-degreesin a direction of movement of the object for regions located on downwardsloping portions of the surface profile, and wherein the angle where therespective region is at the determined working distance is betweenzero-degrees and ninety-degrees in a direction opposite to the directionof movement of the object for regions located on upward sloping portionsof the surface profile.

In yet another case, the beam steering device is selected from a groupconsisting of a mirror galvanometer, a single axis scanner, amicroelectromechanical system (MEMs)-based scanning mechanism, and arotating scanner.

In yet another case, an F-theta lens or telecentric lens is used inconjunction with the optical source.

In yet another case, at least a portion of the object has a modulatingsurface profile.

These and other aspects are contemplated and described herein. It willbe appreciated that the foregoing summary sets out representativeaspects of systems and methods to assist skilled readers inunderstanding the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention will become more apparent in the followingdetailed description in which reference is made to the appended drawingswherein:

FIG. 1 is schematic diagram of an optical coherence tomography (OCT)system, according to an embodiment;

FIG. 2 is a schematic diagram for a computing module, according to thesystem of FIG. 1,

FIG. 3 is a flowchart for a method for surface inspection of an objectusing optical coherence tomography (OCT), according to an embodiment;

FIG. 4A is a diagrammatic side view of a scanner head and object,according to the system of FIG. 1;

FIG. 4B is a diagrammatic side view of the scanner head and object ofFIG. 4A at a later point in time;

FIG. 5A is a diagrammatic side view of a scanner head and object,according to the system of FIG. 1, at a first point in time;

FIG. 5B is a diagrammatic side view of a scanner head and object,according to the system of FIG. 1, at a second point in time;

FIG. 5C is a diagrammatic side view of a scanner head and object,according to the system of FIG. 1, at a third point in time;

FIG. 5D is a diagrammatic side view of a scanner head and object,according to the system of FIG. 1, at a fourth point in time;

FIG. 6 is a diagrammatic side view of a scanner head and object,according to the system of FIG. 1, showing a focal reference plane;

FIG. 7A is an exemplary B-scan in which a defect was detected in a paintlayer of a vehicle part;

FIG. 7B is a plot of a score produced for the exemplary B-scan of FIG.7A;

FIG. 8A is an exemplary B-scan, in which the system determined there areno features present;

FIG. 8B is an exemplary B-scan, in which the system determined there arefeatures present.

FIG. 9 is an exemplary image captured to form a top-level surface viewof an object;

FIG. 10A is an exemplary B-scan of an object without problematic defectsor features;

FIG. 10B is an exemplary A-scan;

FIG. 11 is an exemplary B-scan of an object with problematic defects orfeatures;

FIG. 12 is an exemplary B-scan of an object for determining whetherthere are defects;

FIG. 13 is an exemplary B-scan of an object showing a defect; and

FIGS. 14A and 14B are exemplary C-scans at respectively different anglesof perspective.

DETAILED DESCRIPTION

Embodiments will now be described with reference to the figures. Forsimplicity and clarity of illustration, where considered appropriate,reference numerals may be repeated among the Figures to indicatecorresponding or analogous elements. In addition, numerous specificdetails are set forth in order to provide a thorough understanding ofthe embodiments described herein. However, it will be understood bythose of ordinary skill in the art that the embodiments described hereinmay be practiced without these specific details. In other instances,well-known methods, procedures and components have not been described indetail so as not to obscure the embodiments described herein. Also, thedescription is not to be considered as limiting the scope of theembodiments described herein.

Various terms used throughout the present description may be read andunderstood as follows, unless the context indicates otherwise: “or” asused throughout is inclusive, as though written “and/or”; singulararticles and pronouns as used throughout include their plural forms, andvice versa; similarly, gendered pronouns include their counterpartpronouns so that pronouns should not be understood as limiting anythingdescribed herein to use, implementation, performance, etc. by a singlegender; “exemplary” should be understood as “illustrative” or“exemplifying” and not necessarily as “preferred” over otherembodiments. Further definitions for terms may be set out herein; thesemay apply to prior and subsequent instances of those terms, as will beunderstood from a reading of the present description.

Any module, unit, component, server, computer, terminal, engine ordevice exemplified herein that executes instructions may include orotherwise have access to computer readable media such as storage media,computer storage media, or data storage devices (removable and/ornon-removable) such as, for example, magnetic disks, optical disks, ortape. Computer storage media may include volatile and non-volatile,removable and non-removable media implemented in any method ortechnology for storage of information, such as computer readableinstructions, data structures, program modules, or other data. Examplesof computer storage media include RAM, ROM, EEPROM, flash memory orother memory technology, CD-ROM, digital versatile disks (DVD) or otheroptical storage, magnetic cassettes, magnetic tape, magnetic diskstorage or other magnetic storage devices, or any other medium which canbe used to store the desired information and which can be accessed by anapplication, module, or both. Any such computer storage media may bepart of the device or accessible or connectable thereto. Further, unlessthe context clearly indicates otherwise, any processor or controller setout herein may be implemented as a singular processor or as a pluralityof processors. The plurality of processors may be arrayed ordistributed, and any processing function referred to herein may becarried out by one or by a plurality of processors, even though a singleprocessor may be exemplified. Any method, application or module hereindescribed may be implemented using computer readable/executableinstructions that may be stored or otherwise held by such computerreadable media and executed by the one or more processors.

The following relates generally to imaging and more specifically to asystem and method for steerable focal adjustment for optical coherencetomography.

Optical coherence tomography (OCT), and particularly non-destructiveOCT, is a technique for imaging in two or three-dimensions. OCT canprovide a relatively high resolution, potentially up to few micrometers,and can have relatively deep penetration, potentially up to a fewmillimeters, in a scattering media.

OCT techniques can use back-scattered light from an object to generateinformation about that object; for example, generating athree-dimensional representation of that object when different regionsof the object are imaged.

FIG. 1 illustrates a schematic diagram of an OCT system 100, accordingto an embodiment. The OCT system 100 includes an optical source (orphotonic emitter) 102, a reflective element 104 (for example, a mirror),a beam splitter 110, and a detector (for example, a photodetector) 106.The diagram shows an object 108 with three layers of depth. The opticalsource 102 produces an originating optical beam (or path) 112 that isdirected towards the beam splitter 110. The beam splitter 110 dividesthe originating beam 112 and directs one derivative beam, referred toherein as the reference beam (or path) 114, towards the reflectiveelement 104 and another derivative beam, referred to herein as thesample beam (or path) 120, towards the object to be scanned 108. Bothderivative beams 114, 120 are directed back to the beam splitter 110,and then directed as a resultant beam 118 to the detector 106. In somecases, one or more secondary mirrors (not shown) can be provided toreflect the sample beam 120 onto the object 108. In some cases, theremay be a scanner head 121 to direct the sample beam 120 onto the object108. In some embodiments, as described herein, the scanner head 121 caninclude a beam steering device 129 to direct light to the object 108.The beam steering device may be, for example, a mirror galvanometer inone or two dimensions, a single axis scanner, a microelectromechanicalsystem (MEMs)-based scanning mechanism, a rotating scanner, or othersuitable mechanism for beam steering. The beam steering device may becontrolled electromechanically. In some cases, the scanner head 121 canalso include a depth-of-field adjusting mechanism.

In some cases, the OCT system 100 can include a distance determinationmodule 127 for determining the distance between the scanner head and theobject 108. In an example, the distance determination module 127 can bean infrared line scanner, laser rangefinder, 3D laser scanner, radarbased rangefinder, or the like. In some cases, the distancedetermination module 127 may be associated with, or separate from, thescanner head 121.

In some cases, the system 100 can include an amplification mechanism;for example, a doped fiber amplifier, a semiconductor amplifier, a Ramanamplifier, a parametric amplifier, or the like. The amplificationmechanism can be used to amplify the signal of the optical source 102and/or to increase quantity of photons backscattered off the surfaceunder inspection and collected on the detector 106. By using theamplification mechanism, sensitivity of the system 100 may be increased.

In some cases, the system 100 can include an object translator 109 tomove the object relative to the sample beam 120 and/or the scanner head121. The object translator 109 can be, for example, a conveyor system, arobotic system, or the like. The illustration of FIG. 1 is onlydiagrammatic as the optical paths can be comprised of optical cables,and as such, the system components can have any number of physicalplacements and arrangements.

The optical source 102 can be any light source suitable for use with aninterferometric imaging modality; for example, a laser or light emittingdiode (LED). Particularly, in some implementations, the optical source102 can be a tunable laser the wavelength of which can be altered (i.e.swept) in a controlled manner; for example, to sweep a wide wavelengthrange (e.g. 110 nm) at high speed (e.g. 20 KHz). In a particularexample, the tunable laser can have a centre wavelength of 1310 nm,wherein the wavelength of the emitted light is continuously scanned overa 110 nm range, with a scan rate of 20 kHz and a coherence length ofover 10 mm. In a further embodiment, the optical source 102 may be a lowcoherence light source such as white light or an LED. As an example,using a low coherence light source can facilitate extraction of spectralinformation from the imaging data by distributing different opticalfrequencies onto a detector array (e.g. line array CCD) via a dispersiveelement, such as a prism, grating, or other suitable device. This canoccur in a single exposure as information of the full depth scan can beacquired.

In some cases, the optical source 102 can include a collimator 103 fornarrowing the originating beam 112. In further cases, further optics maybe included in various stages of the system 100 to control or change theoptical beams. Optics may include lenses or other optical devicessuitable to control, guide, navigate, position, or the like, the lightbeam in a desired manner; as an example, an F-theta or telecentric lensmay be included. Where an F-theta or telecentric lens is used, theplanification can mean that the beam steering device 129 only has tocompensate in the axial direction, along the z-axis of the optical beam,as described below.

In further cases, software techniques may be employed for correcting oraffecting optical errors or signals.

The detector 106 can be any suitable photodetector. In a particularcase, the detector 106 can be a balanced photodetector, which can havean increased signal to noise ratio. In further cases, the detector 106may be a photoelectric-type photodetector, such as a charge-coupleddevice (CCD) or complementary metal-oxide semiconductor (CMOS). Thedetector 106 may operate by photoemission, photovoltaic, thermal,photochemical, or polarization mechanism, or other mechanism throughwhich electromagnetic energy can be converted into an electrical signal.Upon receiving the resultant beam 118, the detector 106 can convert theradiance/intensity of the resultant beam 118 into an electrical signal.In some cases, the electrical signal may then be converted to a digitalsignal, and modified by signal conditioning techniques such as filteringand amplification. In some cases, the interference pattern correspondingto the backscattered light can be converted into a signal by thedetector 106 via, for example, a high-speed digitizer.

The OCT system also includes a computing module 200. The computingmodule 200 may be locally communicatively linked or remotelycommunicatively linked, for example via a network 150, to one or moreother elements of the system 100; for example, to the optical source102, the detector 106, the object translator 109, the scanner head 121,the beam steering device 129, and the distance determination module 127.The computing module 200 may be used for processing and analysis ofimaging data provided by the OCT system 100. In some cases, thecomputing module 200 may operate as a control system or controller, andin other cases, may be connected to a separate control system orcontroller. Further, the computing module 200 may host a user-accessibleplatform for invoking services, such as reporting and analysis services,and for providing computational resources to effect machine learningtechniques on the imaging data.

In an embodiment, as shown in FIG. 2, the computing module 200 caninclude a number of physical and logical components, including a centralprocessing unit (“CPU”) 260, random access memory (“RAM”) 264, an inputinterface 268, an output interface 272, a network interface 276,non-volatile storage 280, and a local bus 284 enabling CPU 260 tocommunicate with the other components. CPU 260 can include one or moreprocessors. RAM 264 provides relatively responsive volatile storage toCPU 260. The input interface 268 enables an administrator to provideinput via, for example, a keyboard and mouse. The output interface 272outputs information to output devices, for example, a display orspeakers. The network interface 276 permits communication with othersystems or computing devices. Non-volatile storage 280 stores theoperating system and programs, including computer-executableinstructions for implementing the OCT system 100 or analyzing data fromthe OCT system 100, as well as any derivative or related data. In somecases, this data can be stored in a database 288. During operation ofthe system 200, the operating system, the programs and the data may beretrieved from the non-volatile storage 280 and placed in RAM 264 tofacilitate execution. In an embodiment, the CPU 260 can be configured toexecute various modules, for example, a steering module 290 and ageometry module 292.

In some cases, the system 100 can use machine learning (ML) to transformraw data from the A-scan, B-scan, or C-scan into a descriptor. Thedescriptor is information associated with a particular defect in theobject. The descriptor can then be used to determine a classifier forthe defect. As an example, the CPU 260 can do this detection andclassification with auto-encoders as part of a deep belief network.

OCT systems 100 generally use different localization techniques toobtain information in the axial direction, along the axis of theoriginating optical beam 112 (z-axis), and obtain information in thetransverse direction, along a plane perpendicular to the axis of theoriginating beam 112 (x-y axes). Information gained from the axialdirection can be determined by estimating the time delay of the opticalbeam reflected from structures or layers associated with the object 108.OCT systems 100 can indirectly measure the time delay of the opticalbeam using low-coherence interferometry.

OCT systems that employ low-coherence interferometers can use an opticalsource 102 that produces an optical beam 112 with a broad opticalbandwidth. The originating optical beam 112 coming out of the source 102can be split by the beam splitter 110 into two derivative beams (orpaths). The first derivative beam 114 can be referred to as thereference beam (or path or arm) and the second derivative beam 120 canbe referred to as the sample beam (or path or arm) of theinterferometer. Each derivative beam 114, 120 is reflected back andcombined at the detector 106.

The detector 106 can detect an interference effect (fast modulations inintensity) if the time travelled by each derivative beam in thereference arm and sample arm are approximately equal; whereby “equal”generally means a difference of less than a ‘coherence length.’ Thus,the presence of interference serves as a relative measure of distancetravelled by light on the sample arm.

For OCT, the reference arm can be scanned in a controlled manner, andthe reference beam 114 can be recorded at the detector 106. Aninterference pattern can be detected when the mirror 104 is nearlyequidistant to one of the reflecting structures or layers associatedwith the object 108. The detected distance between two locations wherethe interference occurs corresponds to the optical distance between tworeflecting structures or layers of the object in the path of the beam.Advantageously, even though the optical beam can pass through differentstructures or layers in the object, OCT can be used to separate out theamount of reflections from individual structures or layers in the pathof the optical beam.

With respect to obtaining information in the transverse direction, asdescribed below, the sample beam 120 can be focused on a small area ofthe object 108, potentially on the order of a few microns, andsuccessively scanned over a section of the object 108.

In an embodiment of an OCT system, Fourier-domain can be used as apotentially efficient approach for implementation of low-coherenceinterferometry. Instead of recording intensity at different locations ofthe reference reflective element 104, intensity can be detected as afunction of wavelengths or frequencies of the optical beam 112. In thiscase, intensity modulations, as a function of frequency, are referred toas spectral interference. Whereby, a rate of variation of intensity overdifferent frequencies can be indicative of a location of the differentreflecting structures or layers associated with the object. A Fouriertransform of spectral interference information can then be used toprovide information similar to information obtained from scanning of thereflective element 104.

In an embodiment of an OCT system, spectral interference can be obtainedusing either, or both, of spectral-domain techniques and swept-sourcetechniques. With the spectral-domain technique, the optical beam can besplit into different wavelengths and detected by the detector 106 usingspectrometry. In the swept-source technique, the optical beam producedby the optical source 102 can sweep through a range of opticalwavelengths, with a temporal output of the detector 106 being convertedto spectral interference.

Advantageously, employing Fourier-domain can allow for faster imagingbecause back reflections from the object can be measured simultaneously.

The resolution of the axial and transverse information can be consideredindependent. Axial resolution is generally related to the bandwidth, orthe coherence-length, of the originating beam 112. In the case of aGaussian spectrum, the axial resolution (Δz) can be: Δz=0.44*λ₀ ²/Δλ,where λ₀ is the central wavelength of the optical beam and Δλ is thebandwidth defined as full-width-half-maximum of the originating beam. Inother cases, for spectrum of arbitrary shape, the axial spread functioncan be estimated as required.

In some cases, the depth of the topography imaging for an OCT system istypically limited by the depth of penetration of the optical beam intothe object 108, and in some cases, by the finite number of pixels andoptical resolution of the spectrometer associated with the detector 106.Generally, total length or maximum imaging depth z_(max) is determinedby the full spectral bandwidth λ_(ful) of the spectrometer and isexpressed by z_(max)=(¼N)*(λ₀ ²/λ_(full)) where N is the total number ofpixels of the spectrometer.

With OCT systems, sensitivity is generally dependent on the distance,and thus delay, of reflection. Sensitivity is generally related to depthby: R(z)=sin(p*z)/(p*z)*exp(−z²/(w*p)). Where w depends on the opticalresolution of spectrometer associated with the detector 106. The firstterm related to the finite pixels in the spectrometer and the secondterm related to the finite optical resolution of the spectrometer.

When implementing the OCT system 100, reflected sample and referenceoptical beams that are outside of the coherence length willtheoretically not interfere. This reflectivity profile, called anA-scan, contains information about the spatial dimensions, layers andlocation of structures within the object 108 of varying axial-depths;where the ‘axial’ direction is along the axis of the optical beam path.A cross-sectional tomograph, called a B-scan, may be achieved bylaterally combining a series of adjacent A-scans along an axisorthogonal to the axial direction. A B-scan can be considered a slice ofthe volume being imaged. One can then further combine a series ofadjacent B-scans to form a volume which is called a C-scan. Once animaging volume has been so composed, a tomograph, or slice, can becomputed along any arbitrary plane in the volume.

A-scans represent an intensity profile of the object, and its values (orprofile) characterize reflectance of the way the optical beam penetratesthe surface of the object. Thus, such scans can be used to characterizethe material from the surface of the object to some depth, at anapproximately single region 131 of the object 108. As used in thepresent disclosure, the term ‘surface’, of an object, is understood toinclude the peripheral surface down to the depth of penetration of theA-scan. B-scans can be used to provide material characterization fromthe surface of the object 108 to some depth, across a contour on thesurface of the object 108.

The system 100, as described herein, can be used to detect featuresassociated with the surface and subsurface of an object; and in somecases, for later categorization of such features. In a particular case,such features are defects in the object, due to, for example, variousmanufacturing-related errors or conditions.

In many circumstances, the object to be scanned by the OCT system 100 isnot flat and can have a curved or otherwise modulating surface orprofile. FIGS. 4A and 4B show a portion of an object 408 having a curvedsurface to be scanned; and in some cases, the curved surface may beconsidered monotonic. FIG. 4A illustrates a scanner head 121 scanningthe object 408 at a first instance of time and FIG. 4B shows suchscanner head 121 scanning the object 408 at a later instance of time. Inthis case, the object 408 is moved in the direction illustrated by arrow407 relative to the fixed scanner head 121. The scanner head 121 isdirected at the object 408 and includes the beam steering device 129. Apresent working distance (“WD”) 402 is illustrated as the distancebetween the scanner head 121 or the beam steering device 129 and thesurface of the object 408. A present depth of field 404 (or “confocalparameter” if the beam is assumed to be Gaussian) is also illustratedand represents the distance over which the surface of the object 408 isin focus during the A-scan. Depth of field, as used herein, isunderstood to be a distance about a plane of focus (“POF”) in whichobjects in an image appear sufficiently sharp, and thus, the objects arein focus. In some cases, as exemplified in greater detail in FIG. 6, theupper boundary of the depth of field 404, being the boundary closest tothe scanner head 121, can be called a focal reference plane. The lowerboundary of the depth of field 404, being the boundary farthest from thescanner head 121, can be called the working distance deviation.

As shown in FIG. 4B, as the object is moved along its direction oftravel, the working distance 402 can increase or decrease due to themodulations in the profile of the object 408; in this case, the workingdistance 402 increased as the surface became further away from thescanner head 121 along the axial direction. As shown in FIG. 4B, theincrease was such that the depth of field 404 no longer includes thesurface of the object 408 within operational focus.

One approach to resolve the above problem is to adjust the path lengthof the reference arm 114. In an example, this can include having thereflective element 104 on a motorized mechanism that translates themirror along the axial direction of the reference beam 114 in order toshorten or lengthen the path length. In another example, this caninclude incorporating a liquid lens in the reference path 114 in orderto change the effective reference path length. However, adjusting thepath length may be inadequate for applications where the object ismoving relative to the scanner head, due to computational and speedconcerns.

In the present embodiments, the system 100 can use the beam steeringdevice 129 to change the working distance of the sample beam 120 to keepthe surface of the object 408 in focus and thus adjust for the changesin the profile of the surface of the object 408. As shown in FIGS. 5A to5D, as the object 408 is moved along in direction 407 relative to thescanner head 121, the beam steering device 129 changes the workingdistance 402 of the sample beam 120 to keep the surface of the object408 in focus.

As shown in FIGS. 5A to 5D, the beam steering device 129 steers the beamalong angle denoted by “θ” from the vertical. Then, in some cases, thedepth of field 404 can be adjusted by the depth-of-field adjustingmechanism 125 in order to ensure that the surface of the object 408 iswithin the depth of field.

Referring now to FIG. 3, shown therein is a method 300 of surfaceinspection using the OCT system 100, in accordance with an embodiment.The method 300 may be used for inspecting the surface of an object 408when the object 408 is moved relative to the scanner head 121. In anexemplary case, the method can be used for the purposes of detectingsurface defects or irregularities. The method 300 uses the fact that aworking distance between the scanner head 121 and the surface of theobject 408 is correlated with depth of field 404; whereby the surfacecan be brought into focus by selecting an appropriate working distance.Advantageously, the method 300 accomplishes such working distancemodification by angling of the sample beam 120 by the beam steeringdevice 129, which is relatively quick and efficient, especiallyimportant in circumstances where the object 408 is moved relative to thescanner head 121. Conventional approaches, such as having to linearlyactuate the scanner head in the axial direction, are typically too slowfor such an application. As another intended advantage, the method 300does not require changing of depth of field with a depth of fieldadjusting device. As such, the overall costs and complexity of thesystem 100 can be significantly reduced.

At block 302, the distance determination module 127 determines a surfaceprofile for a first section of the object 408. Each section of theobject's 408 surface includes one or more regions 131 for scanning,whereby each region 131 has at least one A-scan performed at thatlocation, as described herein. In some cases, where the surface profileof the whole object can be determined at block 302, the object 408 canbe considered to comprise one section. Determining the surface profileincludes determining the spatial distance between the scanner head 121and one or more regions 131 on the surface of the object 408; the one ormore regions 131 being located in the first section. The first sectionof the surface is in advance of the scanner head 121 along the surfaceof the object 408 in the direction opposite of the direction of travel407. In this way, the geometry module 292 can make a determination ofthe geometry of the surface of the object 408 for determining workingdistance, as described below.

In an embodiment, the surface profile can be determined by measurementvia one or more distance determination modules 127. While in some casesthe one or more distance determination modules 127 can measure thedistance between the scanner head 121 and the surface of the object 408,generally, the one or more distance determination modules 127 canmeasure the distance between any fixed point and the surface of theobject 408. In further embodiments, distance can be determined by thegeometry module 292 by, for example, retrieving the surface geometry ofthe object 408 from the database 288 or from the input interface 268. Insome cases, the one or more distance determination modules 127 cancomprise the geometry module 292 or perform its functions. As anexample, the surface geometry can be derived from a CAD model of theobject 408. Upon processing the object's surface geometry, the geometrymodule 292 can determine the spatial distance for each region 131 of theobject's 408 surface. In further embodiments, the system 100 can use acombination of distance measurement and predetermined geometry todetermine the spatial distance; for example, using geometric informationto anticipate larger variations in the surface profile by the geometrymodule 292 and utilizing the distance determination module 127 forgreater accuracy. In some embodiments, the measured spatial distances ofthe object 408 can be stored in the database 288 for retrieval by thegeometry module 292 during a subsequent scanning of an identical orsimilar object.

At block 304, the steering module 290 determines, for every region 131in the section of the object's 408 surface, a working distance 402 inwhich the surface is in focus, where in focus meaning that the surfaceis within a present depth of field. In some cases, each region 131 canbe in the range of approximately 100 to 1000 microns apart. In aparticular case, each region 131 is approximately 200 microns apart.

At block 306, for each of the one or more regions, the steering module290 determines a steering angle from the vertical, denoted as “θ” in theexamples of FIGS. 5A to 5D, in which the respective region 131 is at thedetermined working distance 402 away from the scanner head 121.

At block 308, the object 408 is moved relative to the scanner head 121.In further cases, the object 408 can be continuously moved, orrepeatedly moved and stopped.

At block 310, as the object 408 moves relative to the scanner head 121,for each of the one or more regions 131, the steering module 290 directsthe beam steering device 129 to steer the sample beam 120 such that thesample beam 120 is directed at the respective region 131, when suchregion 131 is at the appropriate working distance from the scanner head121 along the respective steering angle θ.

At block 312, for each of the one or more regions, the system 100performs an A-scan, as described herein, when the respective region 131is in focus.

The system 100 repeats blocks 302 to 312 for each successive section ofthe surface of the object 408. The A-scans of method 300 can beaggregated by the computing module 200. The aggregation technique mayinvolve stacking images comprising the imaging data according to imageprocessing techniques. In an embodiment, aggregation of imaging data mayinclude the formation of a B-scan from a plurality of A-scans. In somecases, the B-scan and/or A-scans are presented to a user via the outputinterface 272.

In some embodiments, as shown in FIGS. 5A and 5B, the steering module290 can select steering angles θ that are between the vertical andninety-degrees in the direction of the object's movement 407. In thiscase, the beam steering device 129 can wait for the region 131 to passby the scanner head 121 in order for the sample beam 120 to reach thedesired steering angle θ and therefore working distance 402. In otherembodiments, as shown in FIGS. 5C and 5D, the steering module 290 canselect steering angles θ that are between the vertical andninety-degrees in the direction opposite the object's movement 407. Inthis case, the beam steering device 129 can angle the sample beam 120 atthe region 131 prior to the region 131 passing by the scanner head 121,in order to reach the desired steering angle θ and therefore workingdistance 402. In further embodiments, the beam steering device 129 canangle the sample beam 120 either towards or opposite the object'sdirection of movement 407. As an example, as shown in FIGS. 5A to 5D,the beam steering device 129 can angle the sample beam 120 towards thedirection of movement 407 to focus on regions 131 located on downwardsloping surface sections, and angle the sample beam 120 opposite thedirection of movement 407 to focus on regions 131 located on upwardsloping surface sections.

In method 300, it is generally understood that the surface of the object408 being in ‘focus’ includes approximately the totality of the A-scanaxial depth being in focus (i.e., within the boundaries of the depth offield 404).

Advantageously, the approach described herein does not require that thedepth of field be changed mechanically, which can be costly or complex,but instead only requires the appropriate angling of the sample beam, asthe object is moved, to bring the surface into focus.

In some embodiments, the working distance can be determined by analysisof the A-scan to determine if the object 408 is within the depth offield 404. However, this approach is limited in speed andresponsiveness, especially where the object is moved relative to thescanner head, due to having to analyze the A-scan and then having toretake the A-scan if it is not in focus. Advantageously, the method 300measures the working distance, and brings the surface into focus, priorto the A-scan; such that it is faster and more efficient, and is able toscan an object 408 in movement.

While in the present embodiments the object is described as ‘moving’ viathe object translator 109, it is appreciated that moving can includesuccessively moving and stopping the object 108 for scanning.Additionally, while in the present embodiments, the movement is shownalong a single dimensional axis, it is appreciated that the movement ofthe object can be along a two-dimensional plane.

In some embodiments, the beam steering device 129 may not be capable ofadjusting the sample beam angle quickly enough for each A-scan. In thiscase, the system 100 can take an A-scan at the appropriate speed of thebeam steering device 129 and use digital signal processing or machinelearning techniques to fill in for the missing intermediate A-scans inthe aggregated B-scan.

In some cases, after the A-scans, B-scans, and/or C-scans have beendetermined, the system can detect whether there are defects in theobject using image interpretation and machine learning techniques. Thedefective label indicates that an unacceptable defect has been detected,and in some cases, such defect is of a particular type. In the examplewhere the object is a vehicle part, the defect may have different shapesand dimensions. As an example, the defect may be an unwanted round seedor crater, or the like, on or under the surface of the part. As anotherexample, the defect may have an elongated shape, such as with anunwanted fiber, or the like, on or under the surface of the part. As anexample, the acceptable/defective label may be with regards to the size,area, or volume of a defect. In another example, acceptable/defectivelabel may be with regards to the presence of defect between differentlayers of films applied in an industrial process; for example, in anautomotive setting, in an electro-deposition (ED) layer, a colour layer,or a clear layer, where each layer is in the order of tens of micronsthick.

In some cases, based on analysis of the OCT images, the system 100 canprovide further information in the form of feature localization on theobject. As an example, the information may be that there is fiber defectat location x=3.4 cm, y=5.6 cm on a vehicle part. Feature localizationcan also be specified with respect to surface depth, along the z-axis.Depth localization can be particularly advantageous in certainapplications; for example, when thin films are being applied to avehicle part. In this case, for example, after a vehicle part ispainted, paint inspection may be required on various layers including anelectro-deposition layer, a colour layer, and a clear coat layer. Beingable to detect and determine the presence of a defect between any two ofthese layers is particularly advantageous because it has implications onthe amount of re-work that may be required to resolve the imperfection.It can also be advantageous for improvement to a manufacturing processby being able to determine what type of defect is located at what layer;for example, a faulty HVAC system in the manufacturing environment couldbe responsible for introducing defects between layers. In this regard,being able to localize defect origin to a portion of the manufacturingpath is an advantage to reduce future defects and rework.

The machine-learning techniques described herein may be implemented byproviding input data to a neural network, such as a feed-forward neuralnetwork, for generating at least one output. The neural network may havea plurality of processing nodes, including a multi-variable input layerhaving a plurality of input nodes, at least one hidden layer of nodes,and an output layer having at least one output node. During operation ofa neural network, each of the nodes in the hidden layer applies afunction and a weight to any input arriving at that node (from the inputlayer or from another layer of the hidden layer), and the node mayprovide an output to other nodes (of the hidden layer or to the outputlayer). The neural network may be configured to perform a regressionanalysis providing a continuous output, or a classification analysis toclassify data. The neural networks may be trained using supervised orunsupervised learning techniques. According to a supervised learningtechnique, a training dataset is provided at the input layer inconjunction with a set of known output values at the output layer; forexample, imaging data for which defect location and/or existence isknown. During a training stage, the neural network may process thetraining dataset. It is intended that the neural network learn how toprovide an output for new input data by generalizing the information itlearns in the training stage from the training data. Training may beeffected by backpropagating error to determine weights of the nodes ofthe hidden layers to minimize the error. The training dataset, and theother data described herein, can be stored in the database 288 orotherwise accessible to the computing module 200. Once trained, oroptionally during training, test data can be provided to the neuralnetwork to provide an output. The neural network may thuscross-correlate inputs provided to the input layer in order to provideat least one output at the output layer. Preferably, the output providedby the neural network in each embodiment will be close to a desiredoutput for a given input, such that the neural network satisfactorilyprocesses the input data.

In some embodiments, the machine learning techniques can employ, atleast in part, a long short-term memory (LSTM) machine learningapproach. The LSTM neural network allows for quickly and efficientlyperforming group feature selections and classifications.

In some embodiments, the detection can be by employing, at least inpart, a convolutional neural network (CNN) machine learning approach.

While certain machine-learning approaches are described, specificallyLSTM and CNN, it is appreciated that, in some cases, other suitablemachine learning approaches may be used where appropriate.

As an example, FIG. 7A illustrates a B-scan in which a defect wasdetected in a paint layer of a vehicle part. As shown, the defect iscentered at approximately 225×10⁻² mm along the fast scan axis (x-axis).Correspondingly, FIG. 7B illustrates a plot of a score produced by theCPU 260, between 0 and 1, representing a determined possibility that adefect is present in the exemplary B-scan of FIG. 7A.

As an example, FIG. 8A illustrates a B-scan in which contours areoutlined. In this case, the CPU 260 determined that there was no defectdetected on the object. FIG. 8B also illustrates a B-scan in whichcontours are outlined. In this case, the CPU 260 determined that therewas a defect detected on the object.

FIG. 9 illustrates an exemplary image captured to form a top-levelsurface view of an object.

FIG. 10A illustrates an exemplary B-scan (cross-section) of an objectwithout problematic defects or features (i.e., a ‘clean’ surface). FIG.10B illustrates an exemplary A-scan from the center of the B-scan ofFIG. 10A.

FIG. 11 illustrates an exemplary B-scan (cross-section) of an objectwith a problematic defect or feature present. In this case, as shown,there was a subsurface seed detected, centered at approximately 500along the x-axis.

FIG. 12 illustrates an exemplary B-scan of a vehicle part fordetermining whether there are painting defects. In this case, there wasno defect from the B-scan. FIG. 13 illustrates an exemplary B-scan of avehicle part for determining whether there are painting defects. In thiscase, as shown, there was a defect in the paint layer detected, centeredat approximately 225 along the x-axis.

FIGS. 14A and 14B illustrate, at respectively different angles ofperspective, an exemplary C-scan of a vehicle part. In this case, a seedwas detected as a defect in the painting of a vehicle part.

In further embodiments, machine learning can also be used by the CPU 260to detect and compensate for data acquisition errors at the A-scan,B-scan and C-scan levels.

While the present embodiments refer to the “vertical” as the downwardsdirection, it should be understood that “vertical” can be replaced withany reference angle that is approximately a direct line between thescanner head and the object, whatever orientation such reference anglemay have.

Although the invention has been described with reference to certainspecific embodiments, various modifications thereof will be apparent tothose skilled in the art without departing from the spirit and scope ofthe invention as outlined in the claims appended hereto. The entiredisclosures of all references recited above are incorporated herein byreference.

The invention claimed is:
 1. A method of surface inspection of an objectusing optical coherence tomography (OCT), the method comprising:determining a surface profile of the object, the surface profilecomprising one or more regions on a surface of the object; moving theobject relative to the OCT scanner head; and for each of the one or moreregions on the surface of the object, performing: determining a workingdistance where the surface of the object at the respective region iswithin a present depth of field; determining an angle where therespective region is at the determined working distance from an OCTscanner head; directing the OCT scanner head at the determined angletowards the respective region when the respective region is at thedetermined working distance along the respective angle; and performingan A-scan of the object when the respective region is within the presentdepth of field.
 2. The method of claim 1, wherein moving the objectrelative to the OCT scanner head comprises continuously moving theobject.
 3. The method of claim 1, wherein the one or more regions on thesurface of the object comprise one or more regions in a first section ofthe surface profile, the surface profile comprising at least onesubsequent section, each subsequent section comprising one or moresubsequent regions, the method further comprising, for each one of theone or more subsequent regions, performing: determining a workingdistance where the surface of the object at the respective subsequentregion is within the present depth of field; determining an angle wherethe respective subsequent region is at the determined working distancefrom the OCT scanner head; directing the OCT scanner head at thedetermined angle towards the respective subsequent region when therespective subsequent region is at the determined working distance alongthe respective angle; and performing an A-scan of the object when therespective subsequent region is within the present depth of field. 4.The method of claim 1, wherein at least a portion of the object has acurved surface profile.
 5. The method of claim 1, wherein determiningthe surface profile of the object comprises measuring a plurality ofdistances between a fixed point and the surface of the object.
 6. Themethod of claim 1, further comprising retrieving a surface geometry ofthe object from a database and wherein determining the surface profileof the object comprises determining a distance between the scanner headand the surface of the object from the surface geometry of the object.7. The method of claim 1, wherein the one or more regions areapproximately 100 to 1000 microns away from an adjacent region.
 8. Themethod of claim 7, wherein the one or more regions are approximately 200microns away from the adjacent region.
 9. The method of claim 1, whereinthe angle where the respective region is at the determined workingdistance is between zero-degrees and ninety-degrees in a direction ofmovement of the object.
 10. The method of claim 1, wherein the anglewhere the respective region is at the determined working distance isbetween zero-degrees and ninety-degrees in a direction opposite to adirection of movement of the object.
 11. The method of claim 1, whereinthe angle where the respective region is at the determined workingdistance is between zero-degrees and ninety-degrees in a direction ofmovement of the object for regions located on downward sloping portionsof the surface profile, and wherein the angle where the respectiveregion is at the determined working distance is between zero-degrees andninety-degrees in a direction opposite to the direction of movement ofthe object for regions located on upward sloping portions of the surfaceprofile.
 12. A system for surface inspection of an object using anoptical coherence tomography (OCT) system, the OCT system comprising anoptical source to produce an optical beam, a beam splitter to direct aderivative of the optical beam to a reflective element and anotherderivative of the optical beam to the object via a scanner head anddirect optical beams returned from the reflective element and the objectto a detector for detection of an interference effect, the system forsurface inspection comprising: a distance determination module todetermine a surface profile of the object, the surface profilecomprising one or more regions on a surface of the object; an objecttranslator to move the object relative to the OCT scanner head; and asteering module to, for each of the one or more regions on the surfaceof the object, perform: determining a working distance where the surfaceof the object at the respective region is within a present depth offield; determining an angle where the respective region is at thedetermined working distance from an OCT scanner head; and directing theOCT scanner head at the determined angle towards the respective regionwhen the respective region is at the determined working distance alongthe respective angle, the OCT system performing an A-scan when therespective region is within the present depth of field.
 13. The systemof claim 12, wherein the distance determination module comprises aplurality of measurement devices to measure a plurality of distancesbetween one or more fixed points and the surface of the object togenerate the surface profile.
 14. The system of claim 12, wherein theone or more regions on the surface of the object comprise one or moreregions in a first section of the surface profile, the surface profilecomprising at least one subsequent section, each subsequent sectioncomprising one or more subsequent regions, the steering module, for eachone of the one or more subsequent regions, further performing:determining a working distance where the surface of the object at therespective subsequent region is within the present depth of field;determining an angle where the respective subsequent region is at thedetermined working distance from the OCT scanner head; and directing theOCT scanner head at the determined angle towards the respectivesubsequent region when the respective subsequent region is at thedetermined working distance along the respective angle, the OCT systemperforming an A-scan when the respective region is within the presentdepth of field.
 15. The system of claim 12, wherein the angle where therespective region is at the determined working distance is betweenzero-degrees and ninety-degrees in a direction of movement of theobject.
 16. The system of claim 12, wherein the angle where therespective region is at the determined working distance is betweenzero-degrees and ninety-degrees in a direction opposite to a directionof movement of the object.
 17. The system of claim 12, wherein the anglewhere the respective region is at the determined working distance isbetween zero-degrees and ninety-degrees in a direction of movement ofthe object for regions located on downward sloping portions of thesurface profile, and wherein the angle where the respective region is atthe determined working distance is between zero-degrees andninety-degrees in a direction opposite to the direction of movement ofthe object for regions located on upward sloping portions of the surfaceprofile.
 18. The system of claim 12, wherein the beam steering device isselected from a group consisting of a mirror galvanometer, a single axisscanner, a microelectromechanical system (MEMs)-based scanningmechanism, and a rotating scanner.
 19. The system of claim 12, whereinan F-theta lens or telecentric lens is used in conjunction with theoptical source.
 20. The system of claim 12, wherein at least a portionof the object has a modulating surface profile.